Measuring system with ratiometric frequency output

ABSTRACT

A measuring device, with a digital interface for the transmission of digital signals to an evaluation unit, is disclosed, whereby the interface includes a clock input, to which a system clock signal is fed, a signal input, to which a measured signal is applied, an arithmetic unit, which derives an output signal from the clock signal and measured signal, and a signal output where the output signal is output and transmitted to the evaluation unit. Measurement accuracy may be improved by transmitting a reference signal that is a signal derived from the clock signal, and using this reference signal to correct the output signal.

BACKGROUND OF THE INVENTION

The present invention relates to a measuring system with ratiometricfrequency output and a method for improving the measurement accuracy ofsuch a system.

Measuring systems having one or several sensors and an associatedevaluation unit are widespread. To illustrate the principal design ofmeasuring systems of this type, an air-mass sensor that is known fromautomotive technology will be described hereinbelow in greater detail asan example. The invention should not be limited to air-mass sensors,however.

In order to determine the flow of a flowing medium, such as air,fuel-injection installations include a mass flow meter, which is alsoreferred to in the literature as an air-mass sensor. With knownmass-flow sensors of this type, the sensor element is exposed to astream of air in the intake manifold of the internal combustion engine.The sensor element has a heater and measuring resistors that are cooledvia convection by the air stream, which brings about a change inresistance. The air stream flowing through the intake manifold can bedetermined from the unbalance of a measurement bridge. Finally, thesensor provides a measured signal, which is transmitted to a distantevaluation unit.

For this purpose, an air-mass sensor further includes a (digital)interface for transmission of the measured signal. The evaluation unitextracts the useful information from the received signal and evaluatesit.

A typical example of a measuring system of this type having a digitalinterface is shown in a schematic representation in FIG. 3. Themeasuring system shown includes a measuring device 1 having an interface5 for the transmission of digital signals to an evaluation unit 2.Measuring device 1 and evaluation unit 2 are interconnected via a cable11.

Interface 5 is based substantially on digital circuit technology andincludes a clock input 3, to which a clock pulse having a certainfrequency (e.g., 10 MHz) is supplied, and a signal input 10, to whichthe measured signal from the sensor is applied.

Interface 5 further includes an arithmetic unit 6, which processes themeasured signal and outputs a corresponding signal at signal output 7 ofinterface 5.

The output signal is typically a signal derived from the system clocksignal and the measured signal. As a result, a linear relationshipusually exists between the clock signal and the output signal.

Transmission of the useful information (the measured value) can becarried out basically using all known transmission methods, such asmodulation procedures. The useful information can also be depicted inthe on/off ratio or in the frequency and/or period duration of theoutput signal.

With known systems, the useful signal is usually contained in the periodduration of the output signal, since a coding of this type is relativelyeasy to realize and it enables very high measurement accuracy.

The system clock signal that is applied to clock input 3 is generated bya pulse generator 4, such as an oscillator or quartz. Oscillators orquartzes of this type can have high tolerances and/or pulsefluctuations. The deviation of the system clock signal affects theoutput signal in a directly proportional manner, however, and cantherefore strongly interfere with measurement accuracy.

With applications having a large measuring range, in particular,measuring systems of this type can fulfill the specified requirementsfor measurement tolerance only if highly-accurate quartzes and/oroscillators are used. Precise quartzes are correspondingly expensive,however, and they cannot be integrated directly, due to reasons of cost.

SUMMARY OF THE INVENTION

The object of the present invention, therefore, is to create a measuringsystem that functions with economical quartzes and/or oscillators, yethas narrow measurement tolerances.

The fundamental idea of the invention is to derive an additionalreference signal from the system clock signal and transmit it to theevaluation unit. Said reference signal is used to calculate a correctionfactor that reflects the deviation of the timing frequency from adesired timing frequency and, therefore, the deviation of the frequencyand/or the period duration of the measured output signal from thenon-deviating output signal. This correction factor is taken intoaccount in the evaluation of the output signal, i.e., the frequencydeviation of the reference signal is used to correct the output signal.

The frequency of the reference signal and/or its period duration ispreferably less than that of the clock signal by a factor N, and islocated, in particular, in a range of less than 100 Hz, in particularless than 50 Hz, and preferably at approximately 20 Hz. In contrast, theclock signal has a frequency of 10 MHz, for example.

According to a preferred embodiment of the invention, the interface ofthe sensor has a variable-frequency signal output, i.e., the outputsignal has a different period duration depending on the measured signal.

According to a preferred embodiment of the invention, the evaluationunit includes an arithmetic unit, in particular to calculate thecorrection factor for correction of the output signal from the referencesignal.

The measuring system according to the invention can be used inautomotive technology to optimize fuel injection, for example. In thiscase, the measuring system would include an air-mass meter and anassociated evaluation unit.

To compensate for the temperature course of the sensor curve, theair-mass meter preferably includes a temperature sensor. The valuesmeasured by the temperature sensor are also transmitted to theevaluation unit.

The temperature values are preferably transmitted together with thereference signal. According to a preferred embodiment of the invention,the temperature values are contained in the on/off ratio of thereference signal. The temperature values can also be transmitted viaother transmission paths or by means of other transmission methods,however.

According to a preferred embodiment of the invention, the periodduration of the output signal is quantized in step widths of <=500 nsand, preferably, <=200 ns. The frequency of the output signal ispreferably between approximately 1.5 and 12 kHz.

The transmission of measured values take place according to standardcharacteristic curves, in particular. That is, the measured values arepreferably normalized and are therefore independent of the particulardimensions of the measurement site, such as the intake manifold of anengine.

The characteristic curve of the measuring device is, for example, annth-order polynomial, and, in particular, a third-order polynomial.

The invention will be described in greater detail hereinbelow withreference to the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a principal representation of a measuring system with adigital interface according to an embodiment of the invention;

FIG. 2 shows the interconnection of sensor interface and evaluationunit.

FIG. 3 shows an example of a measuring system of the prior art asdescribed above.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a measuring system that is composed of a measuring device 1and an evaluation unit 2 that are interconnected via a cable 11.

The measuring device 1 includes an interface 5 for the transmission ofdigital signals to evaluation unit 2, whereby interface 5 has a clockinput 3 and a signal input 10.

The clock pulse that is fed to clock input 3 is generated by a pulsegenerator 4, such as a quartz or an oscillator, and has a frequency ofapproximately 10 MHz, whereby the frequency can have relatively highdeviations and/or fluctuations depending on the quality of the pulsegenerator.

Measuring device 1 is based substantially on digital signal processingand outputs a digital measurement signal at interface 5. An arithmeticunit contained in interface 5 processes the supplied clock pulse andmeasured signal and, from them, calculates an output signal, thefrequency and/or period duration of which depends on the measuredsignal. The output signal is therefore a signal with variable periodduration, that is derived from the system clock signal and the measuredvalue. There is a linear relationship, in particular, between the periodduration of the system clock signal and the output signal.

The useful information, i.e., the measured value, that is contained inthe period duration and/or the frequency of the output signal thereforeexhibits the same deviations as the clock signal.

To compensate for these deviations, a reference signal is generated thatis derived from the clock signal. The reference signal is transmittedvia a reference signal output 13 of interface 5 to evaluation unit 2,and has a relatively low frequency of approximately 20 Hz.

The deviation of the period duration of the reference signal is takeninto account in the evaluation of the measured signal. To accomplishthis, an arithmetic unit 9 contained in evaluation unit 2 calculates acorrection factor k, with $k = \frac{T_{desired}}{T_{actual}}$whereby T_(desired) is the value of a desired period duration, andT_(actual) is the period duration of the reference signal that isactually measured. A ratiometric measurement is therefore carried out.

Finally, arithmetic unit 9, taking correction factor k into account,determines the actual measured value, such as an air mass that isflowing through a passage. The characteristic curve used in this exampleis a third-order polynomial, which yields, as the result, the percentageof a maximum air mass m_(max):$\frac{m}{m_{\max}} = {\frac{1}{a} + {\frac{1}{b}\left( \frac{T_{0} - T_{K}}{T_{norm}} \right)} + \left( \frac{T_{0} - T_{K}}{T_{norm}} \right)^{3}}$whereby

a is an absolute portion of the standard characteristic curve,

b is a linear portion of the standard characteristic curve,

TO is a characteristic curve offset,

Tnorm is the characteristic curve range, and

TK is the corrected period duration.

The corrected period duration T_(K) is calculated as follows:TK=k*T _(M)Whereby T_(M) is the period duration that is measured.

FIG. 2 shows the interconnection of interface 5 and evaluation unit 2(engine control device) in detail. Digital interface 5 has a clock input3 and a signal input 10. Interface 5 is configured as an ASIC andfurther has a signal output 12 for a first measured signal (air mass)and a signal output 13 for the reference signal and a second measuredsignal (temperature).

The two signal outputs 12, 13 are each terminated with a resistor R1, R2and a capacitor C1, C2 that is connected to ground. Capacitors C1, C2serve to shunt high-frequency interfering components, in particular tocomply with EMC regulations.

The measured signal for the determination of air mass is transmitted vialine 11, and the measured signal for the determination of temperature istransmitted via line 14 to the engine control device.

The evaluation of the transmitted signal information takes placeaccording to normalized standard characteristic curves, so that theengine control device can determine the physical values for air mass andtemperature using defined arithmetic operations.

The engine control device further includes a pull-up circuit forgeneration of a high level. The pull-up circuit has resistors R3 and R4,one connection each of which is joined with a transmission line 11, 14,and the other connection each of which is connected to a supply voltageU_(pu).

Capacitors C3, C4 that are also connected to ground are provided toeliminate interference from lines 11, 14.

Furthermore, the engine control device includes an RC low-pass filterfor each line 11, 14 to protect a downstream controller (not shown). TheRC low-pass filters have resistors R5, R6 and capacitors C5, C6.

The rise time and fall time of the signal flanks of the measuring deviceare limited, whereby the rise time is determined primarily by the pullupcircuit in the engine control device.

The final stage of the measuring device is protected against the usualenvironmental influences, such as ESD, incident radiation, interferencepulses, etc., and should also be able to withstand faulty operations,such as short circuits.

LIST OF REFERENCE NUMERALS

-   1 Measuring device-   2 Evaluation unit-   3 Clock input-   4 Pulse generator-   5 Interface-   6 Arithmetic unit-   7 Signal output-   8 Reference signal output-   9 Arithmetic unit-   10 Signal input-   11 Cable-   12 Signal output-   13 Reference signal output-   14 Cable-   R1–R6 Resistors-   C1–C6 Capacitors

1. A measuring system comprising a measuring device (1) with a digitalinterface (5) and an evaluation unit (2), wherein the measuring device(1) is a mass flow meter whereby the interface (5) has a clock input(3), to which a pulse generator (4) is connected, a signal input (10),to which a measured signal is applied, an arithmetic; unit (6) thatgenerates an output signal derived from the clock signal and themeasured signal, and a signal output (12), where the output signal isoutput and transmitted to the evaluation unit (2), wherein the interface(5) has a further signal output (13) where a reference signal derivedfrom the clock signal is output, whereby the reference signal is used bythe evaluation unit (2) to correct the output signal.
 2. The measuringsystem as recited in claim 1, wherein the frequency of the referencesignal is less than that of the clock signal by a factor N.
 3. Themeasuring system as recited in claim 1, wherein the output signal has adifferent period duration depending on the measured signal.
 4. Themeasuring system as recited in claim 1, wherein :he evaluation unit (2)has an arithmetic unit (9) for calculating a correction factor tocorrect the output signal.
 5. The measuring system as recited in claim1, wherein the measuring device (1) has a second sensor, and themeasured values tare transmitted to the evaluation unit (2).
 6. Themeasuring system as recited in claim 5, wherein the measured values fromthe second sensor are transmitted together with the reference signal. 7.The measuring system as recited in claim 6, wherein the measured valuefrom the second sensor is contained in the on/off ratio of the referencesignal.
 8. The measuring system as recited in claim 1, wherein themeasuring device (1) is an air-mass meter, and the evaluation unit (2)is an engine control device.
 9. The measuring system as recited in claim1, wherein the period of the output signal is quantized in step widthsof <=500 ns.
 10. The measuring system as recited in claim 1, wherein theevaluation unit (2) has an arithmetic unit (9) that evaluates the outputsignal.
 11. The measuring system as recited in claim 10, wherein thecharacteristic line of the measuring device is an nth-order polynomial.12. The measuring system as recited in claim 1, wherein the frequency ofthe reference signal is <100 Hz.
 13. The measuring system as recited inclaim 1, wherein be frequency of the output signal is between 1.5 and 12kHz.
 14. A method for correcting the output signal of a measuringdevice, being a mass flow meter and having a digital interface, to whicha measured signal and a clock signal are fed, characterized by thefollowing steps: generation of an output signal derived from the docksignal and the measured signal, and transmission of the output signal toan evaluation unit (2); generation of a reference signal derived fromthe clock signal, and transmission of the reference signal to theevaluation unit (2); evaluation of the output signal, whereby deviationsof the output signal are corrected with the aid of the reference signal.15. The method as recited in claim 14, wherein a correction factor iscalculated that represents the deviation of the reference signal from adesired value.
 16. The method as recited in claim 14, wherein the periodduration of the output signal is corrected with the aid of the referencesignal.
 17. The measuring system as recited in claim 1, wherein theperiod of the output signal is quantized in step widths of <=200 ns. 18.The measuring system as recited in claim 10, wherein the characteristicline of the measuring device is a third-order polynomial.
 19. Themeasuring system as recited in claim 1, wherein the frequency of thereference signal is <50 Hz.
 20. The measuring system as recited in claim1, wherein the frequency of the reference signal is approximately 20 Hz.